The present invention relates to the synthesis of silyl camptothecins and silyl homocamptothecins and, particularly, to synthesis of silyl camptothecins and silyl homocamptothecins via a semisynthetic route from camptothecins and homocamptothecins.
References set forth herein may facilitate understanding of the present invention or the background of the present invention. Inclusion of a reference herein, however, is not intended to and does not constitute an admission that the reference is available as prior art with respect to the present invention.
The structure of camptothecin 1a and homocamptothecin 10a are illustrated in FIG. 1. The core structure of the camptothecin class of molecules has five fused rings, A-E. Standard substituents include hydroxyl and ethyl at C20, and other positions of the camptothecin ring core can also be substituted. Homocamptothecin has the same A-D rings as camptothecin, but the E-ring contains an additional methylene group (C20a). The A-B ring system of the camptothecin and homocamptothecin is a quinoline, and this part of the ring system is especially important since substituents in the quinoline part of the molecule often impart useful properties, as detailed below.
In general, camptothecins and homocamptothecins (sometimes referred to generally herein as camptothecins or the camptothecin family) are DNA topoisomerase I inhibitors useful, for example, as anticancer drugs. Analogs of the natural product camptothecin are among the most important classes of compounds available for treatment of solid tumors. Topotecan (tpt) and CPT-11 were the first two members in the camptothecin family to gain United States Food and Drug Administration full approval status (topotecan in 1996 as second-line therapy for advanced epithelial ovarian cancer, topotecan again in 1998 for the treatment of small cell lung cancer, CPT-11 in 1998 as first-line therapy for colon cancer).
Lavergne et al. have shown that expansion of the E-ring of camptothecin to produce a xe2x80x9chomocamptothecinxe2x80x9d enhances the solution stability of camptothecin while maintaining anticancer activity. Lavergne, O., Lesueur-Ginot, L., Rodas, F. P., Kasprzyk, P. G., Pommier, J., Demarquay, D., Prevost, G., Ulibarri, G., Rolland, A., Schiano-Liberatore, A.-M., Harnett, J., Pons, D., Camara, J., Bigg, D., xe2x80x9cHomocamptothecins: Synthesis and Antitumor Activity of Novel E-Ring Modified Camptothecin Analogsxe2x80x9d, J. Med. Chem., 41, 5410-5419 (1998); and Lavergne, O., Lesueur-Ginot, L., Rodas, F. P., and Bigg, D., xe2x80x9cAn E-Ring Modified Camptothecin With Potent Antiproliferative and Topoisomerase I Inhibitory Activities,xe2x80x9d Bioorg. Med. Chem. Lett. 7, 2235-2238 (1997). The modification to the E-ring in the studies of Lavergne et al. involved insertion of a methylene spacer between the carbon bearing the 20-OH functionality and the carboxyl group of the naturally occurring six-membered xcex1-hydroxylactone of camptothecin. Incorporation of the new 7-membered xcex2-hydroxylactone ring into camptothecin has been found to improve the solution stability of the agent. Despite the structural similarity to camptothecins, homocamptothecins behave very differently under physiological conditions. In general, standard camptothecin analogs are dynamic drugs because their lactone rings open rapidly and reversibly under physiological conditions. Typically, the lactone rings of homocamptothecins open comparatively slowly and irreversibly.
7-Silyl camptothecins 2 and 7-silyl homocamptothecins 11 (sometimes referred to as silatecans and homosilatecans) as illustrated in FIG. 1 are important classes of lipophilic camptothecin analogs, See, for example, a) Josien, H.; Bom, D.; Curran, D. P.; Zheng, Y.-H.; Chou, T.-C. Bioorg Med. Chem. Lett., 7, 3189 (1997); b) Pollack, I. F.; Erff, M.; Bom, D.; Burke, T. G.; Strode, J. T.; Curran, D. P. Cancer Research, 59, 4898 (1999); Bom, D.; Du, W.; Garbarda, A.; Curran, D. P.; Chavan, A. J.; Kruszewski, S.; Zimmer, S. G.; Fraley, K. A.; Bingcang, A. L.; Wallace, V. P.; Tromberg, B. J.; Burke, T. G. Clinical Cancer Research, 5, 560 (1999); Bom, D.; Curran, D. P.; Chavan, A. J.; Kruszewski, S.; Zimmer, S. G.; Fraley, K. A.; Burke, T. G. J. Med. Chem., 42, 3018 (1999). Many of the most interesting silatecans and homosilatecans contain one or more additional substituents (for example, hydroxy or amino) in the A ring, and the combination of these substituents can provide significant improvements over either of the corresponding the mono-substituted analogs. For example, 7-tert-butyldimethylsilyl-10-hydroxy camptothecin 2a (DB-67), is currently in late stages of preclinical development. DB-67 and other silatecans and homosilatecans show a number of attractive features including high activity against a broad spectrum of solid tumors, low binding to blood proteins, resistance to lactone opening, high lipophilicity, and potential oral availability among others.
DB-67 and other silatecans and homosilatecans have been prepared by total synthesis using the cascade radical annulation routes. See, for example, U.S. patent application Ser. Nos. 09/007,872, 09/212,178 and 09/209,019, U.S. Pat. Nos. 6,150,343 and 6,136,978, Curran, D. P.; Ko, S. B.; Josien, H. Angew. Chem., Int. Ed. Eng, 34, 2683 (1995) and Josien, H.; Ko, S. B.; Bom, D.; Curran, D. P. Chem. Eur. J., 4, 67 (1998). Those total synthetic routes are highly flexible and allow the preparation of a diverse array of silatecan and homosilatecan analogs by both traditional and parallel routes. However, the total synthesis of silatecans and homosilatecans via cascade radical annulation can require thirteen or more steps and proceeds in about 2% overall yield.
It is very desirable to develop improved synthetic routes for producing silyl camptothecins and silyl homocamptothecins.
The present invention provides semisynthetic routes to the synthesis of silyl camptothecins and silyl homocamptothecins from camptothecins and homocamptothecins by addition of silyl radicals thereto.
In one aspect, the present invention provides a method of synthesizing a compound having the formula 
in racemic form, enantiomerically enriched form or enantiomerically pure form, the method including generally the step of reacting a compound having the formula 
with a silyl radical precursor under conditions to generate a silyl radical. SiR1R2R3 wherein R1, R2 and R3 are, for example, independently the same or different a C1-10 alkyl group, a C2-10 alkenyl group, a C2-10 alkynyl group, an aryl group, xe2x80x94(CH2)mR11, or SiR12R13R14, wherein m is an integer within the range of 1 through 10 and R11 is a hydroxy group, an alkoxy group, an amino group, an alkylamino group, a dialkylamino group, Cl, F, a cyano group, xe2x80x94SRc or a nitro group, and wherein R12, R13 and R14 are independently the same or different an alkyl group or an aryl group. Preferably R1, R2 and R3 are independently the same or different an alkyl group or an aryl group.
R4 and R5 are, for example, independently the same or different hydrogen, xe2x80x94C(O)Rf wherein Rf is an alkyl group, an alkoxy group, an amino group or a hydroxy group, an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, an aryloxy group, an acyloxy group, xe2x80x94OC(O)ORd, wherein Rd is an alkyl group, xe2x80x94OC(O)NRaRb wherein Ra and Rb are independently the same or different, H, xe2x80x94C(O)Rf, an alkyl group or an aryl group, Cl, F, a hydroxy group, a nitro group, a cyano group, an azido group, a formyl group, a hydrazino group, an amino group, xe2x80x94SRc, wherein Rc is hydrogen, xe2x80x94C(O)Rf, an alkyl group or an aryl group; or R4 and R5 together form a chain of three or four members selected from the group of CH, CH2, O, S, NH, or NR15, wherein R15 is a C1-C6 alkyl group. R4 and R5 together can, for example, form a group of the formula xe2x80x94O(CH2)jOxe2x80x94 wherein j represents the integer 1 or 2.
R6 is, for example, H, Cl, F, a nitro group, an amino group, a hydroxy group, or a cyano group; or R5 and R6 together form a chain of three or four members selected from the group of CH, CH2, O, S, NH, or NR5. R5 and R6 together can, for example, form a group of the formula xe2x80x94O(CH2)jOxe2x80x94 wherein j represents the integer 1 or 2.
R7 is, for example, H, F, an amino group, a C1-3 alkyl group, a C2-3 alkenyl group, a C2-3 alkynyl group, a trialkylsilyl group or a C1-3 alkoxy group. R7 is preferably H. R8 is, for example, a C1-10 alkyl group, an alkenyl group, an alkynyl group, or a benzyl group. R8 is preferably an ethyl group, an allyl group, a benzyl group or a propargyl group. Most preferably, R8 is an ethyl group. R9 is, for example, H, F or xe2x80x94CH3, and n is 0 or 1. R10 is, for example, xe2x80x94C(O)Rf or H. Preferably, R10 is H or xe2x80x94C(O)CH3.
Y is either absent or is oxygen. In the case that Y is oxygen, the N-oxide is transformed to the corresponding silyl camptothecin or silyl homocamptothecin during the reaction.
Preferred silyl radical precursors for use in the present invention include silanes, disilanes, silylgermanes, silylstannanes, silyl boranes, and acyl silanes. Several preferred silanes, disilanes, silylgermanes, and silylstannanes can be represented by the general formula XSiR1R2R3 wherein X is H, SiR17R18R19, GeR17R18R19 or SnR17R18R19, respectively, and wherein R17, R18, and R19 are, for example, independently the same of different an alkyl group or an aryl group. Preferred silyl boranes and acyl silanes can also be represented by the formula XSiR1R2R3 wherein X=xe2x80x94B(ORd)2 and X=xe2x80x94C(O)Ri, respectively, wherein Ri is an alkyl group or an aryl group.
In the case of disilanes, silylgermanes, silylstannanes, silyl boranes and acyl silanes, the reaction can be effected by irradiation of the reaction mixture with UV light of a wavelength suitable to cleave the Sixe2x80x94X bond either directly or with sensitization. Additives, as known in the art, for example, sensitizers or (photo-)electron transfer agents, can be present to promote the desired reaction.
In the case of disilanes, the reaction can also be effected chemically by the generation of a reactive radical that will homolytically cleave (by homolytic substitution at silicon) the silicon-silicon bond to generate the silyl radical. Preferred reactive radicals for this method are hydroxy, alkoxy or acyloxyl radicals, and preferred methods of generating these radicals are thermolysis or photolysis of hydrogen peroxide, alkyl hydroperoxides, dialkyl peroxides, alkyl acyl peroxides or diacylperoxides. Other methods of alkoxy radical generation such as thermolysis or photolysis of alkyl hyponitrites are also suitable.
The reaction of silanes (XSiR1R2R3, X=H) can also be effected chemically by the generation of a reactive radical that will homolytically cleave (by homolytic substitution of hydrogen) the silicon-hydrogen bond to generate the silyl radical. Preferred reactive radicals for this method are hydroxy, alkoxy or acyloxy, and preferred methods of generating these radicals are thermolysis or photolysis of hydrogen peroxide, alkyl hydroperoxides, dialkyl peroxides, alkyl acyl peroxides or diacylperoxides. Other methods of alkoxy radical generation such as thermolysis or photolysis of alkyl hyponitrites are also suitable.
Other silyl radical precursors and methods of generation of silyl radicals are known to those skilled in the art. See, for example C. Chatgilialoglu, Chem. Rev. 1995, 95, 1229, the disclosure of which is incorporated herein by reference. Substantially any of these precursors or methods can be used in this invention.
In another aspect, the present invention provides generally a method of synthesizng silyl-quinolines for example, at the C4 position of the quinoline structure) by reacting a quinoline with a silyl radical generator and a silyl radical precursor under conditions to generate a silyl radical. SiR1R2R3. Quinolines have the 
The silyl quinolines of the present invention have the general formula: 
In a preferred embodiment of the silyl radical addition to substituted quinolines, the present invention provides generally a method of synthesizing 7-silyl camptothecins and 7-silyl homocamptothecins including the step of mixing a camptothecins or a homocamptothecin having hydrogen at the C7 position with a silyl radical generator and a silyl radical precursor under conditions to generate a silyl radical xe2x80xa2SiR1R2R3.
The camptothecin precursors in the reactions of the present invention can be obtained from natural sources, by total synthesis by one of several methods, or by modification of existing synthetic or natural camptothecin analogs. The homocamptothecin precursors for the reactions of the present invention can likewise be obtained by semi-synthesis from camptothecin as, for example, described by Lavergne et al., by total synthesis, or by modification of existing homocamptothecin analogs.
Substituents on the camptothecin and homocamptothecin precursors of the present invention (for example, R4, R5, R6, R7, R8, R9, and R10) can be substantially any substituents as known in the art. Substituents on camptothecin and homocamptothecin rings preferably, however, do not react rapidly with silyl radicals of the present invention. In general, reactions of substituents with silyl radicals of the present invention can lead to undesirable byproducts that may be difficult to separate from target products. Examples of suitable substituents include, but are not limited to, R4, R5, R6, R7, R8, R9, and R10 set forth above. Examples of substituents that react rapidly with silyl radicals and are preferably avoided include bromine and iodine.
A number of groups, such as amino groups and hydroxy groups, can be protected using protective groups as known in the art before addition of the silyl radical. Preferred protective groups for hydroxy groups include, but are not limited to, acetate and trimethylsilyl groups. Preferred protective groups for amino groups include, but are not limited to, tert-butyloxycarbonyl, formyl, acetyl, benzyl, p-methoxybenzyloxycarbonyl, trityl. Other suitable protecting groups as known to those skilled in the art are disclosed in Greene, T., Wuts, P. G. M., Protective Groups in Organic Synthesis, Wiley (1991), the disclosure of which is incorporated herein by reference. Such protective groups can be reacted to provide the desired substituent (for example, hydroxy group or an amino group) after addition of the silyl radical using conditions known in the art. In general, protecting groups used in the methods of the present invention are preferably chosen such that they can be selectively removed without affecting the other substituents on the camptothecin ring. In general, 7-silyl substituents on camptothecins (including homocamptothecins) have been found to very stable under a variety of conditions.
Solvents for the reaction can be selected from the full range of traditional organic reaction solvents. Aromatic solvents (for example, benzene and toluene) are less preferred since addition of the silyl radical to the solvent can be a competitive side reaction. Chlorocarbons like chloroform and tetrachloromethane solvents are also less preferred since chlorine abstraction from the solvent by the silyl radical can be a competitive side reaction. However, less reactive chlorocarbons such as 1,2-dichloroethane are more preferred. Other preferred solvents include, ethers (for example tetrahydrofuran (THF), diethylether, dioxane, and the like), alcohols (methanol, ethanol, and the like) and dipolar aprotic solvents (CH3CN, DMF, DMSO, and the like). Water ;need not be excluded from the reaction and can even be used as a cosolvent.
Preferred reaction conditions for generation of silyl radicals from silanes include addition of an organic thiol (R16SH, wherein R16 is, for example, an alkyl group or a trialkylsilyl group). For example, one set of conditions involved heating a mixture of a camptothecin or homocamptothecin analog, a silane (R1R2R3SiH), a peroxide (for example, di-tert-butylperoxide) and a thiol (for example, tert-butane thiol or triisopropylsilane thiol) in an organic solvent. Relative to the camptothecin analog, preferred quantities of the silane range from approximately 1-20 equiv, with approximately 2-10 equiv being more preferred. Preferred quantities of thiol are approximately 0.2-5 equiv, with approximately 1-3 equiv being more preferred. Preferred quantities of the peroxide range from approximately 1-20 equiv with approximately 2-5 equiv being more preferred. Preferred reaction solvents are as discussed above and preferred reaction temperatures range from approximately 60-130xc2x0 C. Lower reaction temperatures can cause reduced conversion to the product while higher temperatures can cause increased amounts of another product in which the silyl group adds to C12 of the camptothecin precursor (if R7=H). A preferred solvent for these reaction conditions is p-dioxane, and a preferred temperature for reactions in this solvent is at or near the reflux point of p-dioxane. Preferred reaction times are 0.5 to 5 days, with more preferred times being 1-2 days. Preferred thiols include, for example, alkane thiols such as dodecane thiol and tert-butane thiol, and trialkylsilanethiols or triarylsilanethiols such as triisopropylsilanethiol. Trialkylsilanethiols or triarylsilanethiols are generally more preferred.
Thiols are sometimes used in radical chain reaction of silanes to assist in chain propagation of the chain reaction in a process that is often called xe2x80x9cpolarity reversal catalysisxe2x80x9d. The silyl additions discovered herein do not appear to be chain reactions, but nonetheless the inventors have discovered that thiols can promote the reaction. The generally mild reaction conditions used with thiols are advantageous in the case of camptothecin and homocamptothecin precursors bearing, for example, acid-sensitive substituents or protective groups.
Silatecans and homosilatecans bearing a 10-hydroxy group have especially interesting biological and chemical properties. The methods of the present invention are well suited for synthesizing such 10-hydroxy silatecans and 10-hydroxy homosilatecans. As the hydroxy group may interfere with the silyl radical generation or addition of prior or subsequent synthetic steps, it may be desirable to use a protecting group as described above or to generate the 10-hydroxy group after addition of the 7-silyl group.
In another aspect, the present invention provides for a compound of the following formula 
wherein R1xe2x80x94R10 are as defined above.
In another aspect, the present invention provides a method for the conversion of a silatecan or homosilatecan lacking any substituent at R5 (that is, R5 is H) to a 10-hydroxysilatecan or 10-hydroxyhomosilatecan analog (that is, R5 is xe2x80x94OH). Converting the hydrogen at R5 to xe2x80x94OH includes generally the step of oxidation to provide an N-oxide, followed by the step of photolysis. The 7-silyl groups of silatecans and homosilatecans have been found to survive the oxidative and acidic reaction conditions used. In a preferred embodiment of this invention, R4, R6 and R7 are also H, and the resulting product is a 10-hydroxysilatecan or homosilatecan.
In this method, the silatecan or homosilatecan is preferably first oxidized to an N-oxide under conditions known to those skilled in the art for this transformation. Preferred conditions are treatment of the silatecan or homosilatecan with hydrogen peroxide in the presence of a carboxylic acid, preferably acetic acid. A mixture of the resulting N-oxide in an organic solvent is then irradiated with light (for example, at the edge of the ultraviolet and visible regions of the spectrum) in the presence of an organic or inorganic acid. Preferred solvents for this reaction are ethers (for example, p-dioxane). The wavelength of irradiated light is preferably in the range of approximately 250-600 nm and more preferably in the range of approximately 275-450 nm. A preferred acid for use in this method is aqueous sulfuric acid in the range of approximately 0.1-5M, preferably approximately 1.0 M, and the preferred amount of the acid relative to the silatecan or homosilatecan is approximately, 1-20 mol equiv, more preferably approximately 1-2 mol equiv.
As indicated above, all compounds of the present invention including the xcex1-hydroxylactone group of silatecans or the xcex2-hydroxylactone group of homosilatecans can exist in racemic form, enantiomerically enriched form, or enantiomerically pure form. The formulas of such compounds as set forth herein cover and/or include each such form.
The terms xe2x80x9calkylxe2x80x9d, xe2x80x9carylxe2x80x9d and other groups refer generally to both unsubstituted and substituted groups unless specified to the contrary. Unless otherwise specified, alkyl groups are hydrocarbon groups and are preferably C1-C15 (that is, having 1 to 15 carbon atoms) alkyl groups, and more preferably C1-C10 alkyl groups, and can be branched or unbranched, acyclic or cyclic. The above definition of an alkyl group and other definitions apply also when the group is a substituent on another group (for example, an alkyl group as a substituent of an alkylamino group or a dialkylamino group). The term xe2x80x9carylxe2x80x9d refers to phenyl or naphthyl. As used herein, the terms xe2x80x9chalogenxe2x80x9d or xe2x80x9chaloxe2x80x9d refer preferably to fluoro and chloro.
The term xe2x80x9calkoxyxe2x80x9d refers to xe2x80x94ORd, wherein Rd is an alkyl group. The term xe2x80x9caryloxyxe2x80x9d refers to xe2x80x94ORe, wherein Re is an aryl group. The term acyl refers to xe2x80x94C(O)Rf. The term xe2x80x9calkenylxe2x80x9d refers to a straight or branched chain hydrocarbon group with at least one double bond, preferably with 2-15 carbon atoms, and more preferably with 2-10 carbon atoms (for example, xe2x80x94CHxe2x95x90CHRg or xe2x80x94CH2CHxe2x95x90CHRg). The term xe2x80x9calkynylxe2x80x9d refers to a straight or branched chain hydrocarbon group with at least one triple bond, preferably with 2-15 carbon atoms, and more preferably with 2-10 carbon atoms (for example, xe2x80x94Cxe2x89xa1CRh or xe2x80x94CH2xe2x80x94Cxe2x89xa1CRh). The terms xe2x80x9calkylene,xe2x80x9d xe2x80x9calkenylenexe2x80x9d and xe2x80x9calkynylenexe2x80x9d refer to bivalent forms of alkyl, alkenyl and alkynyl groups, respectively.
The groups set forth above can be substituted with a wide variety of substituents to synthesize homocamptothecin analogs retaining activity. For example, alkyl groups may preferably be substituted with a group or groups including, but not limited to, a benzyl group, a phenyl group, an alkoxy group, a hydroxy group, an amino group (including, for example, free amino groups, alkylamino, dialkylamino groups and arylamino groups), an alkenyl group, an alkynyl group and an acyloxy group. In the case of amino groups (xe2x80x94NRaRb), Ra and Rb are preferably independently hydrogen, an acyl group, an alkyl group, or an aryl group. Acyl groups may preferably be substituted with (that is, Rf is) an alkyl group, a haloalkyl group (for example, a perfluoroalkyl group), an aryl group, an alkoxy group, an amino group and a hydroxy group. Alkynyl groups and alkenyl groups may preferably be substituted with (that is, Rg and Rh are preferably) a group or groups including, but not limited to, an alkyl group, an alkoxyalkyl group, an amino alkyl group and a benzyl group.
The term xe2x80x9cacyloxyxe2x80x9d as used herein refers to the group xe2x80x94OC(O)Rd.
The term xe2x80x9calkoxycarbonyloxyxe2x80x9d as used herein refers to the group xe2x80x94OC(O)ORd.
The term xe2x80x9ccarbamoyloxyxe2x80x9d as used herein refers to the group xe2x80x94OC(O)NRaRb.
For purification, administration or other purposes, the E-ring (the lactone ring) may be opened with alkali metal such as, but not limited to, sodium hydroxide or calcium hydroxide, to form opened E-ring analogs of compounds of formula (1) as set forth in the compounds of formula (2). The intermediates thus obtained are more soluble in water and may be purified to produce, after treatment with an acid, a purified form of the camptothecin analogs of the present invention.